Bird eggs such as chicken eggs are highly nutritious foods containing various nutritional components. Moreover, eggs are highly suitable for cooking. Owing to these characteristics, eggs are very important food resources. In the process of delivering newly laid eggs to consumers, however, eggshells are sometimes broken (egg breakage) or finely cracked (egg cracking). These broken eggs and cracked eggs have little or no commercial value, which causes an increase in the cost. Owing to the introduction of automatic detectors, the rate of eliminating cracked eggs, which used to range from 7 to 8%, attains 12 to 15% in these days and puts serious pressure on egg farmers' profits.
It is considered that egg breakage and egg cracking arise since eggshells are weakened because of stress or failure in calcium utilization and so on. Calcium participating in eggshell formation is mainly supplied by feeds. Therefore, it is well known that calcium insufficiency in a feed results in thinning and weakening in eggshells.
To overcome the problems of egg breakage and egg cracking, there have been employed methods of 1) shortening the egg collection period; 2) forced molting (stop laying by fasting); and 3) administering an eggshell reinforcing agent.
Examples of the eggshell reinforcing agent as described above in 3) include beef meat-and-bone meal serving as a calcium source, calcium carbonate, CPP (casein phosphopeptide) capable of increasing the absorptivity of calcium (see Patent Document 1), an eggshell reinforcing agent comprising crab shell for reinforcing an eggshell per se together with chitosan, etc. for strengthening the eggshell membrane supporting the eggshell (see Patent Document 2). In addition, there have been known a feed containing poly-γ-glutamic acid (see Patent Document 3) and an eggshell reinforcing agent comprising trehalose (see Patent Document 4).
On the other hand, lactobionic acid represented by O-β-D-galactopyranosyl-(1-4)-D-gluconic acid has been already approved by FDA and added to pudding premixes as a solidifying agent or employed in cosmetics as a humidifier in the United States. Concerning functions, it has been reported that lactobionic acid shows activities of selectively stimulate the proliferation of bifid bacteria (see Patent Document 5) and stimulating mineral absorption (see Patent Document 6). It has been also reported that the administration of a chelate composed of lactobionic acid, iron and copper resulted in an increase in the hemoglobin level and improvement in anemia (see Non-patent Document 1). However, it has never been known that lactobionic acid has an effect of reinforcing eggshells.    Patent Document 1: JP-A-1-285158    Patent Document 2: JP-A-9-47232    Patent Document 3: JP-A-9-28309    Patent Document 4: JP-A-2001-95499    Patent Document 5: JP-A-7-277990    Patent Document 6: JP-A-7-277991    Non-patent Document 1: The American Journal of Clinical Nutrition, 1982, Vol. 36, No. 6, p. 1162 to 1169